In fabrication of devices in recent years, there has been encountered a situation where wet etching treatment for a wafer with a spin etching apparatus is many times employed for purposes to increase a mechanical strength and to improve electric and thermal characteristics of a chip. In wet etching, with increase in the number of wafers processed therein, an etching rate is reduced since circulation use of an etching solution is common. The most important control item in the wet etching is control of a wafer thickness after the etching treatment. The thickness control is preferably conducted using a method for measuring a wafer thickness directly in light of its purpose. There arise cases, however, where patterning has been applied and electrodes have been formed on almost all wafers when a thickness of a wafer is measured, and further where a tape or a glass substrate is adhered on a wafer in order to protect the patterned surface thereof. A method for measuring a thickness with a good precision in such various conditions is very difficult to be conducted or requires a very expensive measuring instrument.